The answer to this question is a bit vague, but that's because this is a roleplay server, which should be the answer to this question.
If you feel that a decent level of roleplay has taken place beforehand, and that either a threat to your life has been made or that you have indeed threatened someone's life and they have not complied with your orders i.e "I'm going to ask you nicely to step out of the truck without a weapon" and they step out with a weapon, then you are able to open fire.
It's very situational, but if you ask yourself each time "has a good level of rp occured here yet?" before you shoot, then you'll find yourself steering clear of questionable actions more often than not.